In many areas of the earth, the shallow part of the ground (e.g., the upper 200 meters) has a very low seismic velocity. When a seismic energy source is activated at or near the surface, a significant amount of energy may stay in this shallow layer, which may effectively act as a wave guide. The energy in this shallow layer can be relatively high in amplitude compared to energy reflected from deeper layers of the earth, and hence acts as noise that may impede an effective seismic investigation of the deeper layers.